BAGHDAD, Iraq — On the last day of campaigning, a roadside bomb killed four American soldiers Tuesday and gunmen assassinated a candidate for parliament in this week’s election. A Shiite politician escaped injury in a bombing south of Baghdad.

The U.S. ambassador, meanwhile, said Tuesday the number of abused prisoners found so far in jails run by the Shiite-led Interior Ministry came to about 120. The statement by Ambassador Zalmay Khalilzad reinforced Sunni Arab claims of mistreatment by security forces — a major issue among Sunnis in the election campaign.

Despite the violence, more than 1,000 Sunni clerics issued a religious decree instructing their followers to vote Thursday, boosting American hopes the election will encourage more members of the disaffected minority to abandon the insurgency.

Three of Iraq’s leading politicians agreed Tuesday that a speedy withdrawal by foreign troops before Iraqi forces are ready would cause chaos.

But the three — former Prime Minister Ayad Allawi, Kurdish leader Massoud Barzani and Sunni Arab politician Tariq al-Hashimi — disagreed on the description of U.S. and other foreign troops. Barzani described them as “forces of liberation,” while al-Hashimi said they were occupiers.

The three leaders, speaking from Baghdad, appeared in a debate on the Dubai-based Al-Arabiya television. Such debates are rare in the Arab world, where candidates mainly rely on rallies attended by hand-picked followers.

Their comments were noteworthy because they represent important constituencies in the Thursday vote, when Iraqis will choose a 275-member parliament to serve for the next four years.

Barzani heads the Kurdish autonomous region in the north and is among the country’s most powerful politicians. Allawi heads a religiously mixed ticket in the Thursday election. Al-Hashimi represents a major Sunni Arab coalition.

Allawi, a secular Shiite, said an early U.S. withdrawal “will lead to a catastrophic war.” And al-Hashimi, whose party has been sharply critical of the U.S. role, said he looked forward to “my country’s liberation.”

Al-Hashimi criticized President Bush for saying the United States is fighting terrorism in Iraq.

“Why should Iraqis pay a bill for something they have nothing to do with?” said al-Hashimi, a candidate for parliament. “Terrorism is not the problem of Iraqis.”

A U.S. military statement said four soldiers from Task Force Baghdad died in a blast northwest of the capital, but did not specify the location. That brought to at least 2,149 the number of U.S. service members to have died since the start of the war in 2003, according to an Associated Press count.

Elsewhere, gunmen killed Sunni Arab candidate Mezher al-Dulaimi as he was filling his car at a gas station in Ramadi, an insurgent stronghold 70 miles west of Baghdad. Al-Dulaimi took part in a conference last month in Cairo that was attended by representatives of Iraq’s major factions.

A prominent Shiite politician, Jalal Eddin al-Sagheer, escaped injury Tuesday when a bomb exploded near his convoy in Latifiyah, about 20 miles south of Baghdad.

The attacks occurred on the second anniversary of the capture of Saddam Hussein, an event hailed at the time as a turning point in an insurgency which actually grew in wake of the arrest.

The Bush administration hopes the election will draw a large turnout among Sunni Arabs and produce a government that can win the trust of the community that is the backbone of the insurgency. That would in turn allow the United States and its coalition partners to begin bringing their troops home next year.

Iraqis living outside the country began voting Tuesday in the United States and 14 other countries.

“We are very happy. This is the day for our generation,” Nusredin Kestay said as he prepared to vote in Nashville, Tenn. “We can talk now and say what we want.”

Ali al-Lami, executive director of the Iraqi Electoral Commission, appealed for peace Thursday, when about 15 million people will be eligible to vote in more than 6,200 polling stations across Iraq.

Many Sunnis boycotted the January election, enabling rival Shiites and Kurds to win most of the parliamentary seats — a development that sharpened communal tensions and fueled the insurgency.

In an encouraging sign, more than 1,000 Sunni clerics issued a religious edict, or a fatwa, on Tuesday urging their followers to vote.

“We hope, God willing, that Iraqis will not miss the opportunity to vote and to avoid being marginalized,” Sheik Ahmed Abdul Ghafour al-Samaraie said on Al-Arabiya television.

While some prominent clerics with links to the insurgency have avoided calling on their followers to vote, the edict is likely to encourage many Sunnis to go to the polls.

“I appreciate the statements made by political and religious leaders calling on Sunni Arabs to participate and on insurgents to cease military operations,” Ambassador Khalilzad told reporters. “I believe that the next government will be more representative.”

On prisoner abuse, the ambassador said “over 100” of the detainees found last month at an Interior Ministry jail in Baghdad’s Jadriyah district were suffering signs of abuse. An additional “21 or 26 people” were found three days ago at another Interior Ministry lockup, he said.

Khalilzad said the United States would “accelerate the investigation” to determine who was responsible for abuses — a longtime Sunni Arab demand.

The Islamic Army in Iraq, a prominent insurgent group, said Tuesday it would not attack polling stations. But it vowed to continue its war against U.S.-led coalition forces.

“To the heroes of the Islamic Army in Iraq: Orders have been issued to avoid polling stations centers to preserve the blood of innocent people,” the statement posted on an Islamist Web site said. However, the group said the order did not signal “our support for the political process.”

On Monday, five Islamic militant groups, including al-Qaida in Iraq, also promised not to try to disrupt the voting, even though it branded the election a “satanic project.”

Coalition and Iraqi forces will be out in strength Thursday to protect voters. Borders and airports have been closed, the nighttime curfew extended and use of private vehicles has been banned during the balloting.

In the northern city of Mosul, bomb-sniffing dogs checked polling stations Tuesday for explosives. Once the sites were deemed secure, Iraqi police took control of the buildings while U.S. troops placed concrete barriers on nearby roads.